This invention relates to auxiliary bearings and more particularly to an auxiliary bearing which cooperates with a main bearing failure indicator.
Shafts in many machines should have the capability of rotating for a period of time after their main supportive bearings have failed. The duration of the previously mentioned period of time depends on the machine's application. For example, when an aircraft is in flight and its generator's main bearings fail, it is desirable that the generator maintain power output for the flight's duration without sustaining further damage. It is also desirable to provide an early failure indication for the main supportive bearings before they have failed completely to allow the machine operator or pilot additional time to evaluate the circumstances and determine a course of action as to whether shutdown of the apparatus or continued operation is called for. Early failure indication for the main bearings permits a scheduled main bearing replacement thus avoiding irreparable damage to the machine while avoiding unnecessary maintenance and exhaustive inspection procedures as is now common for aviation applications. One way to provide simple and reliable bearing failure indication is to allow a shaft mounted runner to abrade insulation from an electrical probe which is inserted through the bearing's housing. The rotating runner normally does not engage the insulated electrical probe but, upon initiation of main bearing failure, radial movement of the shaft and shaft mounted runner is increased until the runner contacts the inserted electrical probe and grounds it to the bearing housing. By grounding the electrically charged probe, a circuit is completed and an electrical alarm indicator is signaled. However, this necessitates a runner which will wear away the insulation quickly and yet provide a long-lasting auxiliary bearing.
Secondly, auxiliary bearings often fail when exposed to debris emanating from the main bearing after its failure. The optimum solution to such a problem is to physically remove the auxiliary bearing from the vicinity of the main bearing but, this may be impossible in lightweight, compact applications such as aircraft generators. An alternative to physical removal is shielding the auxiliary bearing from the main bearing.
If the auxiliary bearing is shielded from main bearing debris, the auxiliary bearing may also be shielded from lubricating fluid supplied to the main bearing. Also, in oil-cooled bearing designs, most main bearing failures are due to lack of lubricating fluids supplied thereto. Since lubrication systems are frequently used as the oil source for both main bearings and auxiliary bearings, any failure of those lubrication systems would adversely affect the auxiliary bearings as well as the main bearings.
Many bearing wear indicators have been developed such as U.S. Pat. No. 3,108,264; U.S. Pat. No. 3,102,759; and U.S. Pat. No. 3,897,116. The aforementioned patents, however, provide no auxiliary bearings and have wear indicators of greater complexity and lower reliability than the present invention. U.S. Pat. Nos. 3,183,043 and 3,508,241 by Creeger et al and Potter, respectively, illustrate bearing wear indicators used in combination with auxiliary type bearings. The auxiliary bearings in those patents are drastically different from those of the present invention and have less favorable wear and journaling characteristics than this invention. The auxiliary bearings which are the subject of Potter's patent have no positive lubrication means illustrated or described for them. In fact, Potter's patent indicates the rotating machine should be disconnected from its driving source upon failure of the main bearings so as to allow the machine to come to rest while being supported by the auxiliary bearings. A disadvantage of Potter's auxiliary ball bearings is that the ball bearings experience fretting corrosion during normal operation when the auxiliary ball bearings are not engaged. Since there are no known simple inspection procedures for auxiliary ball bearings they are often unnecessarily replaced during overhaul of mechanisms such as aircraft generators.
Creeger's auxiliary bearing is simply a guiding surface on the main bearing housing structure which will journal a shaft when the main bearing fails. The guiding surface for the auxiliary bearing, even with lubrication supplied thereto, will experience a temperature increase upon main bearing failure which can cause the auxiliary bearing to exhibit unfavorable wear characteristics resulting in improper shaft support after a relatively short time.